Atopic Dermatitis

Description

Atopic dermatitis is a heritable disorder in which animals are hypersenstive to common environmental allergens. It is one of the most common skin diseases of dogs worldwide.

The most common allergens involved in atopic dermatitis are those of house dust mites (Dermatophagoides farinae) and grain mites. Human and animal danders, house dust, grass and tree pollen and moulds also frequently incite reactions. Susceptible animals produce allergen-specific IgE to these normally-innocuous allergens, which then binds to receptors on cutaneous mast cells. Atopic animals may have defects in the epidermis, leading to impaired barrier function, and it is thought that further allergen exposure occures via percutaneous absorption. This further exposure gives mast cell degranluation, releasing istamine, cytokines, chemokines, leukotrienes, prostaglandins and other chemical mediators. This is a type 1 (immediate) hypersensitivity reaction¹, and leads to vasodilation, inflammatory cell infiltration and pruritus. It appears that the cytokines involved are abnormally biased towards a Th2 response. IL-4 in particular is produced; this cytokine is responsible for B-cell production of IgE. Bacterial superantigens and autoantigens released due to keratinocyte damage play a role in perpetuating inflammation.

Signalment

Atopic dermatitis is a disease of dogs, although it can occur sporadically in the cat¹. The typical age of onset of atopic dermatitis is between 6 months and 3 years of age and signs are hardly ever seen in animals under 6 months of age. Signs may be so mild at first thtat they are not noted, but usually progress to become clinically apparent¹. Atopy is heritable and so breed predispoitions occur. Susceptible breeds include the : Beaceron, Boston Terrier, Boxer, Cairn Terrier, Cocker Spaniel, Dalmation, English Bulldog, English Setter, Fox Terrier, Sealyham Terrier, Setters, Shar-Pei, West Highland White Terrier, Wire Hiared fox Terrier, and Yorkshire Terrier^beale. Certain breeds such as the Cocker Spaniel, Dachshund, Doberman Pinscher, German Shepherd, German Short-haired Pointer and Poodle appear to have a decreased risk of atopy. There is no sex predilection.

Diagnosis

The diagnosis of atopic dermatitis is based on the signalment, athorough history and appropriate physical examination findings. Other causes of pruritus must also be ruled out. The differential diagnoses are¹:

• Food allergy: ruled out by a dietary exclusion trial.
• Flea allergy dermatitis: ruled out observing response to ectoparasiticides.
• Contact dermatitis: ruled out by confining animal to one area covered in e.g. newspaper.
• Scabies:ruled out observing response to ectoparasiticides.

Intradermal and serologic allergy testing are available but are not used to make a diagnosis of atopy. Their purpose is to identify the specific offending allergens in an animal in order than immunotherapy may be pursued. The results of allergy testing are only significant if the history and clinical signs are also suggestive of atopic dermatitis.

Clinical Signs

Signs are often, but not always, seasonal. Pruritus is the hallmark of atopic dermatitis and may be the only complaint. This gives rise to self-trauma, causing lesions. Lesions commonly include alopecia, erythema, scaling, crusting, excoriations and salivary staining. Macular-papular eruptions are occasionally seen^beale. With time, lichenification, and hyperpigmentation develops. Because the route of allergen contact is thought to be percutaneous absorption^beale, it follows that hairless regions are most frequently affected: the face, ears, axillae, feet and inguinal regions are predilection sites. Secondary infections such as superficial staphylococcal pyoderma and Malassezia are common, and otitis externa often occurs concurrently^{merck, beale, willemse}. A small number of cases exhibit only chronic or recurrent otitis externa. Another uncommon presentation is allergic rhinitis, manifesting as sneezing, nasal discharge or allergic conjunctivitis^beale.

Laboratory Tests

Routine haematology and biochemistry rarely show any abnormalities in dogs, but eosinophilia is often seen in cats¹.

Serum based allergy tests are widely used in the evaluation of canine atopic dermatitis. However, they are often used prior to ruling out other disorders with similar clinical signs (scabies, food allergy/intolerance). Measurement of total serum IgE levels is not a useful tool for diagnosing atopic dermatitis in dogs. IgE levels are not significantly different in dogs with atopic dermatitis than in normal healthy dogs. In addition, things like routine vaccinations and the presence of ecto or endoparasites may influence serum IgE levels. Finally, there may be breed variations in serum IgE levels.

Allergen-specific IgE measurement may be used to identify allergens for immunotherapy once a clinical diagnosis of atopic dermatitis has been established. In these assays, the patient's serum is evaluated for IgE antibody directed against a panel of regional allergens (pollens, mold, dust, epidermals). Methodologies vary between laboratories, but in general, the principal is the same: the serum is allowed to react with an individual allergen extract . The unreacted antibodies are washed away, and then the allergen-bound antibody is detected using an IgE specific reagent (coupled to an enzyme or radioisotope). The IgE bound reagent is then quantified which should be proportional to the amount of allergen-specific IgE. Measurement depends on the assay utilized, but may use radiometric, fluorometric, or colorimetric measures.

Unfortunately, the laboratories are not subject to any standardization or quality control procedures. Reliability and repeatability of some of these commercial tests are unacceptable. Because there is no external regulation of these diagnostic laboratories, we must rely on the ethics of the companies performing the tests.

Besides laboratory inaccuracies, there are other factors which may influence test results: age, season during which the patient is tested, prior or current corticosteroid therapy.

Treatment

cyclosporin

References

1. Tilley, L P and Smith, F W K (2004) The 5-minute Veterinary Consult (Fourth Edition),Blackwell.
2. Beale, K M (2006) Atopic Dermatitis: Clinical Signs and Diagnosis. Proceedings of the North American Veterinary Conference 2006.
3. Merck & Co (2008) The Merck Veterianry Manual (Eight Edition), Merial.
4. Willemse, T (2007) The Newest on Canine Atopic Dermatitis. Proceedings of the Southern European Veterinary Conference & Congreso Nacional AVEPA.